The process of manufacturing certain electronic devices, such as semiconductor integrated circuits, liquid crystal displays and the like, requires the use of high-resolution projection optical systems. In such systems, a pattern on a photomask or "reticle" is illuminated with light from a source of illumination. The light passing therethrough is imaged onto a workpiece, such as a light-sensitive substrate (e.g., as a silicon wafer coated with photoresist) by a projection lens. Due to the increasing degree of integration of electronic devices, the resolution demanded by the projection optical system has steadily increased. To meet this demand, it is necessary for the projection lens of the projection optical system to operate at a shorter wavelength light and/or to have a larger numerical aperture (NA).
Shortening the operating wavelength of the projection optical system to the ultraviolet (UV) region of the electromagnetic spectrum has the consequence that a limited number of optical materials are available for use. For example, for wavelengths of light under 300 nanometers (nm), synthetic quartz and fluorite (calcium fluoride) are the only glass types that have suitable transmission properties. Unfortunately, the Abbe number of these glasses are close to one another, so it is difficult to compensate for the various aberrations in the projection lens, including chromatic aberration, for lens designs that employ only these glass types.
In contrast, reflective optical systems have no chromatic aberration. Thus, various projection lenses have been proposed which have both reflective and refractive lens elements (i.e., "catadioptric" lenses). Certain proposed high-resolution catadioptric projection lenses include a beam splitter for folding the optical path and have been disclosed in Japanese patent application Kokoku No. Hei 7-117648, Japanese patent application Kokai No. Hei 6-300973, Japanese patent application Kokai No. Hei 5-88089, Japanese patent application Kokai No. Hei 3-282527, PCT/EP95/01719, and U.S. Pat. No. 5,241,423.
The projection lenses disclosed in the above-identified patent applications have optical axes associated with the lens elements before to (i.e., upstream of) the beam splitter element and after (i.e., downstream of) the beam splitter element that are not parallel. However, the recent demands for higher NA, as well as larger field size, require that the size of the refractive elements and the reflective element be increased. Unfortunately, increasing the size of these lens elements is problematic because of the deformation effects of gravity when the projection lens is mounted in the projection optical system. When the optical axes of the refractive elements upstream and downstream of the beam splitter are not parallel, asymmetric deformations arise in the lens elements due to gravity. These deformations induce aberrations which unacceptably reduce the resolution of the projection lens. Unfortunately, these aberrations are such that they cannot be readily corrected during manufacturing.